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#' Converts a date to a decimal of its year.
#'
#' @export decimal_date
#' @S3method decimal_date default
#' @S3method decimal_date zoo
#' @S3method decimal_date its
#' @param date a POSIXt or Date object
#' @return a numeric object where the date is expressed as a fraction of its year
#' @keywords manip chron methods
#' @examples
#' date <- ymd("2009-02-10")
#' decimal_date(date) # 2009.11
decimal_date <- function(date)
UseMethod("decimal_date")
decimal_date.default <- function(date){
if(any(!inherits(date, c("POSIXt", "POSIXct", "POSIXlt", "Date"))))
stop("date(s) not in POSIXt or Date format")
decimal <- as.numeric(difftime(date, floor_date(date, "year"),
units = "secs"))/as.numeric(difftime(ceiling_date(date,
"year"), floor_date(date, "year"), units = "secs"))
year(date) + decimal
}
decimal_date.zoo <- function(date)
decimal_date(zoo::index(date))
decimal_date.its <- function(date)
decimal_date.default(attr(date, "dates"))
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